1. Field of the Invention
The present invention relates in general to optical non-contact sensor technology and more particularly to an interrogation system which uses a single-fiber launch/receive system for interrogating a biosensor to detect the occurrence of a bio-chemical interaction (e.g., biological binding of ligands with analytes).
2. Description of Related Art
Many areas of biological research today utilize optical non-contact sensor technology to help perform increasingly sensitive and time-constrained assays. In such assays, an optical interrogation system can be used to monitor changes in the refractive index or variations in the optical response of an optical sensor (biosensor) as a biological substance is brought into a sensing region of the optical sensor. The presence of the biological substance alters the optical response of the optical sensor when it causes a bio-chemical interaction like material binding, adsorption etc . . . This alteration of the optical response enables one to use the optical sensor to directly monitor biological events in label-free assays where the expense and experimental perturbations of fluorescent dyes are completely avoided.
Inherent in this type of optical interrogation system is the need to use a launch/receive system to launch the light that interacts with the optical sensor, and to subsequently receive the output of the optical sensor to enable the interpretation of the sensor's response. While a launch/receive system that utilizes free-space optics provides the most direct control of the optical signals, the launch/receive system that utilizes optical fiber has many desirable properties. For example, the launch/receive system that uses optical fiber is immune to dust and dirt, does not need to use many expensive bulk optical components, and has the ability to create an arbitrary light path which allows complete flexibility in the location of the light source, the optical sensor, and the light detector.
However, one of the main drawbacks of a launch/receive system that uses optical fiber is the difficulty and poor efficiency of coupling light into the fiber core. For instance in the case of optical sensors, coupling must generally occur at two different places: the light source and the sensor output. At the light source, the problem is usually mitigated by the availability of prepackaged optical fiber light sources. But, the sensor output poses a much more challenging task. Unless specifically integrated into the optical fiber, the typical optical sensor does not have the cylindrical geometry necessary to output a mode similar to the target waveguide of the optical fiber. As such, the coupling efficiency from the optical sensor into the fiber is poor, and large-area multimode fibers are sometimes employed to alleviate this problem. Furthermore, the sensor often has input and output ports that are spatially separated, or at least not completely coincident in space (slightly different coupling angle or location). This means that the launch fiber is often precluded from being the receive fiber, even if the loss from coupling back into a singlemode fiber is tolerable. Thus, the traditional launch/receive system typically requires the use of two fibers (one each for the launch and receive functions), and furthermore may require two different types of fiber: multimode at the receive end for maximum light collection, and singlemode at the launch end in order to have well-defined, consistent operation of the optical sensor. As an example, consider a grating-coupled waveguide (GCW) optical sensor, described in many places in the literature such as in an article by K. Tiefenthaler et al. entitled “Sensitivity of Grating Couplers as Integrated-Optical Chemical Sensors”, J. Opt. Soc. Am. B 6, 209–220 (1988). It is well known that the GCW optical sensor requires a light beam with a well-defined, single-longitudinal mode spatial profile as an input, while the output mode of the GCW optical sensor is less powerful, not spatially well-defined, emerges at a complimentary angle to the input beam, and is often spatially shifted from the input beam. Not surprisingly, the literature references that employ optical fiber as the GCW optical sensor interface describe the use of two separate fibers (or at least fiber cores, packaged into the same cladding or jacket) to provide for the dual launch/receive functionality. For example, see the article by B. Cunningham, P. Li, B. Lin, and J. Pepper, “Colorimetric Resonant Reflection as a Direct Biochemical Assay Technique”, Sensors and Actuators B 81, 316–328 (2002). The contents of this article and the previous article are incorporated by reference herein.
Unfortunately, since the traditional multiple-fiber launch/receive system requires two separate optical fibers it also has a lot of complexity due to the sensitive alignment of the two optical fibers or the manufacture of specialized integrated optical devices such as gradient index (GRIN) lens collimators, fiber alignment chucks, or fused/lensed fiber systems. Accordingly, there is a need for a single-fiber launch/receive system that can address the aforementioned shortcomings and other shortcomings of the traditional multiple-fiber launch/receive system. These needs and other needs are satisfied by the single-fiber launch/receive system of the present invention.